Method of coating metals with amine phosphate coating and composition therefor



Unite States Patent lVIETHOD OF COATING METALS WITH AMINE PHOSPHATECOATING AND COMPOSITION THEREFOR William S. Russell, Royal Oak, Mich.,assignor to Parker Rust Proof Company, Detroit, Mich., a corporation ofMichigan No Drawing. Application March 26, 1952, Serial No. 278,742

14 Claims. (Cl. 148-616) This invention concerns the treatment ofsurfaces of mild steel, zinc, and aluminum for the purpose of producingthereon a protective coating without the necessity of pretreatment ofthe metal surface. The invention concerns particularly the coating ofsteel, zinc, and aluminum by the chemical action of a phosphatecontaining bath on the metal surface wherein the phosphate is an aminephosphate.

This application is a continuation-in-part of my prior copendingapplication, Serial No. 209,323, filed February 3, 1951, for AminePhosphate Coating Solutions, now abandoned.

It is common practice in the art of metal coating to treat metalsurfaces with acidic solutions of phosphates where the phosphate may bephosphoric acid or a salt of phosphoric acid. It is also known that inparticular instances an organic phosphate may be used in a nonaqueoussolution and in other instances may be used to provide a source ofphosphoric acid by the decomposition of the organic phosphate at anelevated temperature. In connection with organic phosphates, forexample, Prutton in U. S. Patent No. 2,224,695 discloses that esters andester salt derivatives of phosphoric acid may be used to form a coatingof an organic iron phosphate on the surface of a metal where the esteris used in a non-aqueous media. Further, he discloses that the presenceof moisture accelerates the reaction of such organic phosphates in theformation of an organic iron phosphate coating.

An essential feature of the present invention lies in the use of anaqueousmedium and the formation of a metal phosphate coating rather thanan organic metal phosphate coating.

Phosphates have been classified as coating phosphates or noncoatingphosphates depending on whether the cation associated with the phosphatebecomes an essential constituent of the coating or whether the phosphateformed is substantially that of the metal coated. The

difference between these two coating mechanisms can become veryimportant from an economic point of view when the cation associated withthe phosphate is derived from a relatively expensive source. Again usingthe above-mentioned patent by way of example, the coating formed is anorganic iron phosphate. Therefore, the phosphate was used as a coatingphosphate and the concentration of organic material would be depletedconstantly during use. It is a feature of the present invention that anorganic phosphate is used as a non-coating phosphate.

An additional serious problem which is encountered in this art concernsthe nature of the metal surface to be treated. The effectiveness of thebest coating baths known is impaired considerably when the metalsurfaces are dirty or contain finger marks, grease and other residuesnot normally soluble in the coating bath. Thisproblern has been solvedto some extent by pretreatment of the metal surfaces with greasesolvents, and blasting, etc. In general, however, the effectiveness ofcoating baths has been linked closely with the effectiveness of metalpretreatment.

It is, therefore, an object of this invention to provide a compositionfor the forming of an adherent metal phosphate coating on the surface ofmild steel, zinc, or aluminum without requiring the extensivepretreatment of the metal surface now found necessary.

It is a further object of this invention to provide a preliminarycleaning coating for mild steel, zinc, or aluminum which can be readilyconverted to the standard phosphate coatings of the art or can be givena standard chromic acid rinse to form in both cases excellent paintbases without the necessity of the extensive pretreatment now requiredfor the formation of these later coatings.

It is also an object of this invention to provide for the formationof ametal phosphate coating in acne-step process; to provide for theformation of such a coating to be followed up by the use of a standardcoating phosphate bath in a two-step process, or by a chromic rinse fora two-step process; or to provide for the formation of such a coating tobe followed up by both the use of a standard coating phosphate bath andchromic rinse for a three-step process where the one-, two-, andthree-step processes produce a superior result in both cleaning andcoating the metal and providing excellent paint bases and drawing andwear-resistant surfaces.

It has now been found that the foregoing and related objects may beachieved with an aqueous coating bath which is maintained at a pHbetween 3 and 6 and which comprises as the principal coating-formingingredient a heterocyclic amine phosphate wherein nitrogen is the solehetero atom which yields at least about .01 percent phosphoric acid inthe coating bath. Improved quality coatings may be obtained by adding tosuch aqueous solutions, controlled proportions of certain oxidizingagents.

The preferred heterocyclic amine phosphates forthe purposes of thisinvention are pyridine and substituted pyridine phosphates having thefollowing structure:

wherein R is a ring substituent selected from the class consisting ofhydrogen and hydrocarbon radicals having less than 7 carbon atoms and ofthe group consisting of alkyl, aryl and cycloalkyl, and wherein each ofthe foregoing hydrocarbon radicals may contain hydroxyl, bromo, c-hloro,sulfo and nitro substituent groups. It is tobe understood that the leadline attaching R to the ring is illustrative only and is, intended inits conventional sense to connect R to the ring; that is, R may beattached to any one of the S'carbon atoms in the ring. The pyridinephosphates above shown may be used in quantities yielding at least about.01 of a phosphate ion up to quantities which saturate the solution. Thepreferred concentration is about .05 to .15 phosphate ion.

Particularly good results have been obtained with dine having no R groupring substituent.

In aqueous solution, it will be understood that the amine phosphates 0fthe above given structure are in actuality in the form of ions ofpyridine or substituted pyridine in equilibrium with phosphate ions suchthat the heterocyclic amine may occupy any one of the three displaceablehydrogen positions of the phosphoric acid radical. When one of thehydrogens of the phosphoric acid radical is substituted by oneheterocyclic amine group, the resulting aqueous solution has a pH ofapproximately 4.2. When 2 of the displaceable hydrogens of thephosphoric acid radical are substituted by heterocyclic aminesfthe pH ofthe aqueous solution approaches 7. As indicated hereinabove,theoperating range of pH of the-solutions of this invention isrestricted to between 3 and 6. It will lie-understood, therefore, thatin order to achieve a pH of 3 in an aqueous solution of heterocyclicamine phosphates having the above given structure, it is necessary thatthere be a slight excess of free phosphoric acid in the solution.Solutions having a pH of about 6 contain a mixture of monoaminephosphates and diamine phosphates or in other Words, the solutioncontains a slight excess of amine over that required to replace 1hydrogen ion, but less than is required to fully replace 2 hydrogen ionsof each phosphoric acid radical.

An amine phosphate ionizes very rapidly in aqueous media to yield theequilibrium proportions of the amine phosphate, phosphoric acid andamine. Similarly, phos phoric acid reacts rapidly in aqueous media withthe same amine to yield the same equilibrium proportions. Therefore, thephosphate may be added as a mixture of the equivalent proportions ofphosphoric acid and heterocyclic amine of the above given structure. Inmost instances where the pH of a solution is to be corrected downwardly,it is desirable to add phosphoric acid to serve the dual purpose ofincreasing the acidity, and of increasing the concentration of phosphateions. Similarly, and where the pH is to be corrected upwardly, an alkalimetal phosphate, carbonate, oxide, hydroxide or the like is mostsuitable. From the foregoing, it will be apparent that the compositionof the solutions and the relative proportions of heterocyclic amines andphosphates which are contained therein are specifically dependent uponthe recited pH.

Coatings of exceptionally good quality may be obtained from thesolutions of this invention when there is incorporated therein limitedquantities of certain oxidizing agents. Although the presence ofoxidizing agents is not necessary in order to achieve the objects of theinvention, particularly adherent and paint receptive coatings resultfrom the use of an oxidizing agent and this coating may be improved evenfurther by the subsequent treatment of the coating with a chromic rinse.Preferred oxidizing agents and the operating ranges of concentrations inthe solutions of this invention are as follows:

Other oxidizing agents which may be employed but which are somewhatinferior to the above preferred oxidizing agents for commercial reasonsinclude sodium meta nitrobenzene sulfonate in concentrations of .l to 4percent and hydrogen peroxide in concentrations of .001 percent to .05percent. Within the above given ranges, the specific percentage for eachof the various oxidizing agents which is most suitable from thestandpoint of quality as well as economy may vary considerably, but theoptimum percentage is quite easily determined in each specific instancesince the improved coatings resulting from the use of oxidizing agentsare in each instance heavier coatings than are produced with the aminephosphate itself. This statement should not be interpreted as implyingthat there is any relationship between coating weight and quality ofcoating, for actually there is not. In this specific case, however, theimproved quality coatings derived by the adding of an oxidizing agentare heavier coatings and, thus, the coating weight may be taken as areference standard to determine the proper percentage of oxidizing agentto yield an improved quality coating.

The composition of the present invention may be painted or sprayed onthe metal surface, or the metal may be immersed in a bath of the coatingcomposition. The temperature of the solution and the time of contactbetween metal and solution is not critical. Temperatures of about 130l70F. are most suitable for coating metal in a relatively short time buttemperatures down: to room temperature can be used. Visible coatings areformed on both steel and zinc, but the coating on aluminum, although anexcellent paint base, cannot be seen by visual inspection. The aluminumphosphate coating is probably transparent because of its extremethinness.

As indicated previously, an advantage is gained from the use of thecompositions of this invention in those cases in which it is desired toform on the surface-one of the well-known phosphate coatings such asisproduced with a zinc phosphate solution. When zinc phosphate coatings orother metallic phosphate coatings produced from coating-phosphatesolutions are desired, the zinc phosphate solution may be applieddirectly to the metal which has been preliminarily coated in accordancewith the method of the present invention. In many instances thepreliminarily formed coating will. dissolve in the coatlug-phosphatebath and be replaced by the phosphate coating from the second bathbecause the latter baths are normally run at quite high acidities. Theimproved coating which results from such a two-step method, appears toowe its quality to the nature of the surface which resulted from thepreliminary treatment in accordance with the method of this invention.That is, the use of the composition of the present invention not onlycoats the metal but thoroughly cleans it and provides an excellent basefor an additional or alternative coating.

The phosphate coatings produced by the method'of this invention as wellas phosphate coatings of the conventional zinc phosphate type resultingfrom the above described two-step method, may be further improved bytreating the coatings with a hexavalent chromium rinse in accordancewith conventional methods and with chromic acid concentrations which arestandard practice in this art.

The following examples are given to more clearly illustrate the methodand the solutions of this invention. In the below given examples theterm point solution refers to the milliliters of one-tenth normal sodiumhydroxide which is required to neutralize a 10 ml. sample of thesolution to the phenolphthaleiu endpoint.

Example 1 A series of solutions was made up as follows:

a. 40 grams pyridine and 23.5 ml. 75% phosphoric acid were made up to 6liters with water, giving a pH of 5.4;v

b. Pyridine and phosphoric acid were combined with water to produce a 20point solution having a pH of 3.0;

c. 47 grams methyl pyridine and 23.5 ml. 75% phosphoric acid were madeup to 6 liters with water, giving a pH of 5.4;

a.- 62 grams, 2 methyl, 5 ethyl pyridine and 23.5 ml. 75 phosphoric acidwere made up to 6 liters with water giving a pH of 5.4;

e. 54 grams dimethyl pyridine and 23.5 ml. 75% phosphoric acid were madeup to 6 liters with water giving a pH of 5.4;

f. 78.5 grams of phenyl pyridine and 23.5 ml. 75% phosphoric acid weremade up to 6 liters with water, giving a pH of 5.4;

g. The pH of solution b was altered by the addition of varyingproportions of pyridine to produce pH values between 3 and 6.

Each of the preceding solutions at a temperature of 160 F. was sprayedon the surface of a cold rolled mild steel panel for a period of oneminute, the steel panels having received no preliminary treatment. Thecoatings produced were very adherent and varied in color from grayishwhite to dark gray with an outside thin, light tan, dusty film. Thecoating weight varied from approximately 15 to mg. per square foot ofsurface area, and in each instance was essentially iron phosphate. Thecoatings were then rinsed with hexavalent chromium solutions inaccordance with standard practice and the resulting coatings were foundto be excellent paint bases.

Example 2 Each of the solutions described in Example 1 was modified byadding thereto varying proportions of chlorate, nitrite, bromate andsulfite oxidizing agents within the ranges of concentrations hereinaboveindicated, and the processing carried out in the manner indicated underExample 1. Definite improvement was noted in each of the coatings, andthe coating weights were in each case substantially greater than thecorresponding examples without the oxidizing agent.

Example 3 Solutions having the compositions indicated under Example 1were sprayed upon the surface of a cold rolled mild steel panel at atemperature of 160 F. for one minute. The treated panels were thensubjected to spraying by a conventional zinc phosphate coating solutioncontaining a nitrate accelerator at 160 F. for about one minute.

The zinc phosphate coatings produced were rinsed with a standardhexavalent chromium solution. The coatings produced were found to haveexcellent corrosion resistance, resistance to humidity and to be anespecially satisfactory base for paint.

What is claimed is:

1. A composition for coating a metal of the class consisting of mildsteel, zinc and aluminum comprising an aqueous solution consistingessentially of phosphoric acid, water, and a substituted heterocyclicamine phosphate having the structure:

ture:

wherein R is a ring substituent selected from the class consisting ofhydrogen and hydrocarbon radicals having less than 7 carbon atoms and ofthe group consisting of alkyl, aryl and cycloalkyl, the pH of thesolution being between about 3 and 6, said solution containing at least.01% phosphate ion.

3. A composition for coating a metal of the class consisting of mildsteel, Zinc and aluminum comprising an aqueous acidic solutionconsisting essentially of a heterocyclic amine phosphate whereinnitrogen is the sole hetero atom which yields at least about .0l% of aphosphate ion in the said solution, the pH of the solution being betweenabout 3 and 6.

4. A composition for coating a metal of the class consisting of mildsteel, zinc and aluminum comprising an aqueous acidic solutionconsisting essentially of a heterocyclic amine phosphate whereinnitrogen is the sole hetero atom which yields at least about .0170 of aphosphate ion in the said solution, and a small amount of an oxidizingagent, the pH of the solution being between about 3 and 6.

5. A composition for coating a metal of the class consisting of mildsteel, zinc and aluminum comprising an aqueous acidic solution having apH between about 3 and 6 and consisting essentially of pyridinephosphate which yields at least about .01% of a phosphate ion in thesaid solution. p

6. A method for cleaning and coating a metal of the class consisting ofmild steel, zinc and aluminum which comprises the step of contacting thesurface of the metal with an aqueous solution consisting essentially ofa heterocyclic amine phosphate, wherein nitrogen is the sole hetero atomwhich yields at least about .01% of a phosphate ion in the solution, thesaid solution having a pH of between about'3 and 6 7. A method forcleaning and coating a metal of the class consisting of mild steel,zincand aluminum which comprises the step of contacting the surface ofthe metal with an aqueous solution consisting essentially of aheterocyclic amine phosphate wherein nitrogen is the sole hetero atomwhich yields at least about .01% of a phosphate ion in the solution, anoxidizing agent in an amount sufiicient to result in an increase incoating weight, the said solution having a pH between about 3 and 6.

8. A method for cleaning and coating a metal of the class consisting ofmild steel, zinc and aluminum which comprises the step of contacting thesurface of the metal with an aqueous solution consisting essentially ofa heterocyclic amine phosphate wherein nitrogen is the sole hetero atomwhich yields at least about .01% of a phosphate ion in the solution, anoxidizing agent in an amount sufficient to result in an increase incoating weight, the said solution having a pH between about 3 and 6 andthereafter rinsing the coated surface with a hexavalent chromiumsolution.

9. A method for forming a phosphate coating on the surface of a metal ofthe class consisting of steel, zinc and aluminum which comprises thesteps of contacting the surface of the metal with an aqueous acidicsolution having a pH of about 3 to 6 and consisting essentially of aheterocyclic amine phosphate wherein nitrogen is the sole hetero atomwhich yields at least about .01% of a phosphate ion in the solutionwhereby the said solution chemically reacts with the metal surface toform an inorganic phosphate coating thereon, thereafter contacting thecoated surface with an aqueous acidic phosphate solution consistingessentially of a phosphate of the class of coatingphosphates to therebyform a second phosphate coating on the surface.

10. A method for forming a phosphate coating on the surface of a metalof the class consisting of steel, zinc and aluminum which comprises thesteps of contacting the surface of the metal with an aqueous acidicsolution having a pH of about 3 to 6 and consisting essentially of aheterocyclic amine phosphate wherein nitrogen is the sole hetero atomwhich yields at least about .01% of a phosphate ion in the solutionwhereby the said solution chemically reacts with the metal surface toform an inorganic phosphate coating thereon, thereafter contacting thecoated surface with an aqueous acidic phosphate solution consistingessentially of a phosphate of the class of coating-phosphates to therebyform a second phosphate coating on the surface, and thereafter rinsingthe coated surface with a hexavalent chromium solution.

11. A composition for coating a metal of the class con.- sisting of mildsteel, zinc and aluminum comprising an aqueous solution consistingessentially of phosphoric acid, water, and a substituted heterocyclicamine phosphate having the structure:

wherein R is a ring substituent selected from the class consisting ofhydrogen and hydrocarbon radicals having less than 7 carbon atoms and ofthe group consisting of alkyl, aryl and cycloalkyl, and wherein each ofthe foregoing hydrocarbon radicals contain at least one substituentselected from the group consisting of hydroxyl, brorno, chloro, sulfo,and nitro, the pH of the solution being between about 3 and 6, saidsolution containing at least .01 phosphate ion.

13. A method for cleaning and coating a metal of the class consisting ofmild steel, zinc and aluminum which comprises the step of contacting thesurface of the metal with an aqueous solution consisting essentially ofphosphoric acid, water, a small amount of an oxidizing agent, and asubstituted heterocyclic amine phosphate having the structure:

g g N between about 3 and 6,-said solution containing at least .01%Phosphate ion.

14. A method for cleaning and coating a metal of the class consisting ofmild steel, zinc and aluminum which comprises the step of contacting thesurface of the metal with an aqueous solution consisting essentially ofphos phoric acid, water, a small amount of an oxidizing agent, and asubstituted heterocyclic amine phosphate having the structure:

wherein R is a ring substituent selected from the class consisting ofhydrogen and hydrocarbon radicals having less than 7 carbon atoms and ofthe group consisting of alkyl, aryl and cycloalkyl, and wherein each ofthe foregoing hydrocarbon radicals contain at least one substituentselected from the group consisting of hydroxyl, bromo, sill f0, andnitro, the pH of the solution being between about 3 and 6, said solutioncontaining at least .01% phosphate 1011.

References Cited in the file of this patent UNITED STATES PATENTS1,805,982 Gravell May 19, 1931 2,318,606 Goebel et al. May 11, 19432,416,734 Boggs et a1. Mar. 4, 1947 2,456,947 Jernstedt Dec. 21, 1948'2,479,423 Snyder Aug. 16, 1949 2,552,874 Snyder et al. May 15, 19512,609,308 Gibson Sept. 2, 1952 OTHER REFERENCES Beilstein: Handbuch derOrganischen Chernie, 4th edition, published Berlin, vol. 20, page 191(1935).

1. A COMPOSITION FOR COATING A METAL OF THE CLASS CONSISTING OF MILDSTEEL, ZINC AND ALUMINUM COMPRISING AN AQUEOUS SOLUTION CONSISTINGESSENTIALLY OF PHOSPHORIC ACID, WATER, AND A SUBSTITUTED HETEROCYCLICAMINE PHOSPHATE HAVING THE STRUCTURE: